We prove an upper bound for the ground state energy of a confined gas of N bosons, optimal up to errors vanishing as N tends to infinity. We consider particles moving on the three-dimensional unit torus, interacting through a hard sphere potential with radius of order 1/N (Gross-Pitaevskii regime). This is joint work with G. Basti, S. Cenatiempo, A. Olgiati and G. Pasqualetti.